1. Field of the Invention
The invention relates to an apparatus for generating an FSK-modulated optical signal for an optical transmission system having an optical FSK super-heterodyne receiver.
2. Description of the Prior Art
In standard optical receivers having super-heterodyne reception, such as optical heterodyne receivers, fluctuation of the amplitude of the intermediate frequency signal is dependent upon the relative position of the polarization conditions of the optical data signal and of the optical signal of a local oscillator. For optimum transmission quality, these fluctuations must be kept as low as possible. Common solutions to this problem are polarization follow-up with a control loop, as disclosed in Electron. Lett. 22 (1986), pp 1341-1343; polarization quadrature reception, Electron. Lett. 22 (1987), pp 168-169; and polarization scrambling, Electron. Lett. 23 (1987), pp 513-514.
The receivers having polarization follow-up theoretically achieve high sensitivity. However, the outlay for the control circuitry in such receivers is considerably large.
Polarization quadrature receivers represent a reliable alternative, but require a division of the optical signals into two sub-signals that are polarized so that they are orthogonal to one another. Each of the sub-signals requires its own receiver.
Both the polarization follow-up and polarization quadrature methods demand an increased outlay in the receiver. This drawback is problematical in many receiver applications.
In polarization scrambling, as opposed to polarization receivers, the increased outlay can be shifted onto the transmitter side. In this method, the polarization condition of the data signal is repeatedly switched between orthogonal polarization stakes in a statistical manner during the bit duration. This allows for the mean value of different polarization conditions to be represented by the intermediate frequency signal. Theoretically, the sensitivity loss in this method compared to the ideal polarization follow-up amounts to at least 3dB. For higher data rates, extremely fast polarization switches are required and the required receiver band width is significantly increased.
It is well-known to have different polarization conditions of an FSK-modulated optical signal that are orthogonal to one another. As in light waveguide links, orthogonal polarization conditions or states remain largely preserved over relatively long optical transmission paths. As a result, polarization-insensitive reception is possible. Such an apparatus is disclosed in Electron. Lett. 24 (1988), pp. 358-360.
The known method for generating the FSK-modulated optical signal, which exhibits the two polarization conditions that are different from one another, is sending the FSK-modulated, polarized optical transmission signal through a highly birefringent medium. After passing therethrough, the two optical frequencies of the FSK-modulated optical transmission signal, the first corresponding to a bit status of 0 and the second corresponding to a bit status of 1, are orthogonal to one another. This condition will only be met for a certain value of the difference between these two frequencies. For this reason, the frequency shift .DELTA.f must be selected large in comparison to the data rate so that any deviation from the orthogonality will be negligible within the signal bandwidth. In the known method, a frequency shift of .DELTA.f-1 GHz is selected for a data rate of only 50Mbit/s. This prior art apparatus is limited because it is only suitable for an optical transmission system having an optical two-filter FSK super-heterodyne receiver.